Memory cell device and programming methods

ABSTRACT

A memory device including a memory cell comprising phase change material is described along with methods for programming the memory device. A method for programming disclosed herein includes determining a data value for the memory cell, and applying a pulse pair to store the data value. The pulse pair includes an initial pulse having a pulse shape adapted to preset the phase change material in the memory cell to a normalizing resistance state, and a subsequent pulse having a pulse shape adapted to set the phase change material from the normalizing resistance state to a resistance corresponding to the determined data value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent application Ser. No. 11/777,195 filed on 12 Jul. 2007, which application claims the benefit of U.S. Provisional Application 60/888,149, filed 5 Feb. 2007, entitled Memory Cell Device Programming Method, both of which are incorporated by reference herein.

PARTIES TO A JOINT RESEARCH AGREEMENT

International Business Machines Corporation, a New York corporation; Macronix International Corporation, Ltd., a Taiwan corporation, and Infineon Technologies A.G., a German corporation, are parties to a Joint Research Agreement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high density memory devices based on programmable resistive or other memory material, like phase change based memory materials, and to methods for programming such devices.

2. Description of Related Art

Chalcogenide materials are widely used in read-write optical disks. These materials have at least two solid phases including a generally amorphous phase and a generally crystalline phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.

Chalcogenide materials also can be caused to change phase by application of electrical current. This property has generated interest in using programmable resistive material to form nonvolatile memory circuits.

In phase change memory, data is stored by causing transitions in an active region of the phase change material between amorphous and crystalline states using current. Current heats the material and causes transitions between the states, and is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation.

It has long been recognized that multiple resistive levels can be achieved according to the level of crystallization within a phase change material. See, for example, Ovshinsky, “Method and Apparatus for Storing and Retrieving Information,” U.S. Pat. No. 3,530,441, issued Sep. 20 2, 1970; and Flynn, “Phase Change Data Storage Device for Multi-Level Recording,” U.S. Pat. No. 6,899,938, issued May 31, 2005. However, the conventional methods for achieving multi-level programming with phase change memory devices have not been completely successful because the distribution of the resistance values associated with each data value are larger across an array of memory cells than is desirable. That is, the resistance level corresponding to a given data value stored in a memory cell using conventional techniques varies from memory cell to memory cell in an array more than is desirable.

Accordingly, it is desirable to provide a multi-level programming method having a reduced distribution of resistance values associated with each data value.

SUMMARY OF THE INVENTION

A memory device including a memory cell comprising phase change material is described along with methods for programming the memory device.

A method for programming described herein includes determining a data value for the memory cell, and applying a pulse pair to store the data value. The pulse pair includes an initial pulse having a pulse shape adapted to preset the phase change material in the memory cell to a normalizing resistance state, and a subsequent pulse having a pulse shape adapted to set the phase change material from the normalizing resistance state to a resistance corresponding to the determined data value.

A memory device described herein includes a memory cell comprising phase change material, and bias circuitry adapted to apply a bias arrangement to the memory cell for storing a data value. The bias arrangement for storing the data value comprises a pulse pair, the pulse pair including an initial pulse having a pulse shape adapted to preset the phase change material in the memory cell to a normalizing resistance state and a subsequent pulse having a pulse shape adapted to set the phase change material in the memory cell from the normalizing resistance to a resistance corresponding to the data value.

An aspect of the present invention of programming a data value for a memory cell includes applying an initial pulse or a sequence of pulses to cause a transition of the active region of the phase change material into “normalizing” phase, for example a generally amorphous phase. This transition of the active region “normalizes” the condition of the memory cell and helps to make the resulting resistances for each level more uniform, thus tightening the distribution of the resistances for a memory device having an array of memory cells.

Other aspects and advantages of the technology described herein can be understood with reference to the figures and the detailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 are simplified cross-sectional views illustrating five example configurations for a memory cell having a programmable phase change material coupled to first and second electrodes.

FIGS. 6-9 illustrate a method for programming a memory cell in accordance with an embodiment.

FIG. 10 is a graph illustrating resistance value distribution ranges for the phase change material of a memory cell programmed by the method illustrated in FIGS. 6-9.

FIG. 11 is a simplified block diagram of an integrated circuit in accordance with an embodiment.

DETAILED DESCRIPTION

The following description of the invention will typically be with reference to specific structural embodiments and methods. It is to be understood that there is no intention to limit the invention to the specifically disclosed embodiments and methods but that the invention may be practiced using other features, elements, methods and embodiments. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations of the description that follows. Like elements in various embodiments are commonly referred to with like reference numerals.

A detailed description is provided with reference to FIGS. 1-11.

FIG. 1 is a simplified cross-sectional view illustrating a first configuration for a memory cell 16 having a programmable phase change material 10 coupled to first and second electrodes 12, 14. A dielectric spacer 13 having a width 15 separates the first and second electrodes 12, 14. The memory material 10 extends across the dielectric spacer 13 and contacts the first and second electrodes 12, 14, thereby defining an inter-electrode path between the first and second electrodes 12, 14 having a path length defined by the width 15 of the dielectric spacer 13. In operation, as current passes between the first and second electrodes 12, 14 and through phase change material 10, a portion of the phase change material 10 called the active region 18 heats up more quickly than the remainder of the phase change material 10. One of the design considerations for phase change devices is to minimize the size of active region 18 thereby reducing the power required to reset the phase change material 10. The size and location of active region 18 is determined in part by the insulating properties of the materials (not shown) surrounding the phase change material 10 and the current path between electrodes 12 and 14.

FIG. 2 is a simplified cross-sectional view illustrating a second configuration for a memory cell 26 having a programmable phase change material 20 coupled to first and second electrodes 22, 24. The phase change material 20 has an active region 28 and contacts the first and second electrodes 22, 24 at top and bottom surfaces 23, 29 respectively. The phase change material 20 has a width 21 the same as that of the first and second electrodes 22, 24.

FIG. 3 is a simplified cross-sectional view illustrating a third configuration for a memory cell 36 having a programmable phase change material 30 coupled to first and second electrodes 32, 34, the phase change material 30 having an active region 38. The first and second electrodes 32, 34 are separated by dielectric spacer 35. The first and second electrodes 32, 34 and the dielectric spacer 35 have a sidewall surface 31. The phase change material 30 is on the sidewall surface 31 and extends across the dielectric spacer 35 to contact the first and second electrodes 32, 34.

FIG. 4 is a simplified cross-sectional view illustrating a fourth configuration for a memory cell 46 having a programmable phase change material 40 coupled to first and second electrodes 42, 44. The phase change material 40 has an active region 48 and contacts the first and second electrodes 42, 44 at top and bottom surfaces 43, 49 respectively. The phase change material 40 has a width 41 less than that of the first and second electrodes 42, 44.

FIG. 5 is a simplified cross-sectional view illustrating a fifth configuration for a memory cell 56 having a programmable phase change material 50 coupled to first and second electrodes 52, 54. The first electrode 54 has a width 51 less than width 53 of the second electrode 52. Because of the difference between width 51 and width 53, in operation the current density in the phase change material 50 is largest in the region adjacent the second electrode 54, resulting in the active region 58 having a “mushroom” shape as shown in the Figure.

It will be understood that the present invention is not limited to the example configurations illustrated in FIGS. 1-5, and additional configurations for memory cells will be apparent to those skilled in the art. For additional information on the manufacture, component materials, use and operation of memory cell devices, in particular phase change random access memory devices, see U.S. patent application Ser. No. 11/155,067, filed 17 Jun. 2005, entitled Thin Film Fuse Phase Change Ram And Manufacturing Method.

The present invention recognizes that it would be desirable to be able to reliably program the active region of memory cells to three or more phases, for example a generally amorphous phase and at least two different crystalline phases, thereby increasing the amount of data that can be stored in a given memory cell and thus increasing the data storage density of a an array of memory cells.

The phase change material of the memory cell is programmable to a plurality of different data values. The different data values (data levels) typically include one value corresponding to a generally amorphous phase for the active region of the phase change material, and the remaining values corresponding to crystalline phases each having different ratios of amorphous to crystalline structure within the active region. In some embodiments, one of the crystalline phases is essentially entirely crystalline so as to maximize the difference in resistance from that of an amorphous phase. Because a crystalline structure of the phase change material has a much lower electrical resistance than an amorphous structure, the data value of memory cell can be determined by the resistance exhibited by the phase change material.

FIGS. 6-9 illustrate a method for programming a memory cell in accordance with an embodiment, the memory cell comprising programmable phase change material capable of being programmed to one of three or more different data values. In the programming method illustrated in FIGS. 6-9 the programmable phase change material is programmed to one of four different resistance values depending upon the determined data value to be stored in the memory cell. However, it will be understood that the scope of the present invention includes programming the phase change material of a memory cell to one of three or more different data values.

FIG. 10 is a graph illustrating resistance states 1000, 1010, 1020, 1030 for the phase change material of the memory cell programmed by the method illustrated in FIGS. 6-9. The resistance states 1000, 1010, 1020, 1030 each have resistance distribution ranges, and each resistance state 1000, 1010, 1020, 1030 corresponds to one of the four data values (two bits) for the memory cell programmed by the method illustrated in FIGS. 6-9. For example, resistance state 1000 can correspond to a memory cell data value of “00”, resistance state 1010 can correspond to a data value of “01”, resistance state 1020 can correspond to a data value of “10”, and resistance state 1030 can correspond to a data value of “11”. Alternatively, the resistance states 1000, 1010, 1020, 1030 can correspond to any of the other data values as will be apparent to those skilled in the art. In general, the number of resistance states depends upon the number of values being programmed, each resistance state corresponding to a different data value.

FIG. 6 illustrates programming a first data value to the memory cell if the determined data value is the first data value by applying a reset pulse 600. The reset pulse 600 causes a transition of an active region of the phase change material into an amorphous phase, thereby setting the phase change material to a resistance within the resistive value range of resistance state 1000 of FIG. 10. The reset pulse 600 is a relatively high energy pulse, sufficient to raise the temperature of at least the active region above the transition (crystallization) temperature 620 of the phase change material and also above the melting temperature 630 to place at least the active region in a liquid state. The reset pulse 600 being for example, between about 1 ns to about 50 ns long, and as another example being between about 20 ns and about 500 ns. In some embodiments the reset pulse results in a voltage difference of between about 2V and about 5V across the phase change material of the memory cell. The reset pulse is then terminated, resulting in a relatively quick quenching time 610 as the active region quickly cools to below transition temperature 620 so that the active region stabilizes to an amorphous phase. Quenching time 610 is typically less than about 10 ns and preferably less than about 1 ns.

The programming of the first data value can also be accomplished using more than one pulse, for example using a pair of pulses. Using a pair of pulses for the programming of the first data value can be desirable for timing purposes when the programming of the other data values also use a pair of pulses, for example like those illustrated in FIGS. 7-9.

FIG. 7 illustrates programming a second data value to the memory cell if the determined data value is the second data value by applying a pulse pair comprising applying a pulse 700 and applying a pulse 720. Applying the pulse 700 causes a transition of the active region into an amorphous phase to normalize the phase change material to resistance state 1000 of FIG. 10, and applying the pulse 720 causes a transition of a first portion of the active region into a crystalline phase, thereby setting the phase change material to a resistance within the resistive value distribution range of resistance state 1010 of FIG. 10. Preferably pulse 700 is similar to reset pulse 600.

The pulse 720 is sufficient to raise the temperature of the first portion of the active region above transition temperature 620 such that the first portion transitions into the crystalline phase. The length of time 730 between the completion of the quenching and the beginning of the pulse 720 is preferably small so that the memory cell 16 can be quickly programmed, the time 730 typically being at least about 5 ns, and preferably at least about 1 ns.

FIG. 8 illustrates programming a third data value to the memory cell 16 if the determined data value is the third data value by applying a sequence of pulses comprising applying a pulse 800 and applying a pulse 820. Applying the pulse 800 causes a transition of the active region 18 into an amorphous phase to normalize the phase change material to resistance state 1000 of FIG. 10, and applying the pulse 820 causes a transition of a second portion of the active region 18 into a crystalline phase, thereby setting the phase change material 10 to a resistance within the resistive value distribution range of resistance state 1020 of FIG. 10. Preferably pulse 800 is similar to reset pulse 600.

In the illustrated embodiment the pulse 820 has a pulse length of time larger than that of the pulse 720 of FIG. 7, and is sufficient to raise the temperature of the second portion of the active region 18 above transition temperature 620 such that the second portion transitions into the crystalline phase. The larger pulse length results in the second portion of the phase change material that transitions into the crystalline phase being greater than the first portion, thus lowering the resistance of the phase change material of the memory cell to the resistance state 1020 of FIG. 10. The length of time 830 between the completion of the quenching and the beginning of the pulse 820 is preferably small, and in some embodiments is equal to that of the time 730 of FIG. 7.

FIG. 9 illustrates programming a fourth data value to the memory cell 16 if the determined data value is the fourth data value by applying a sequence of pulses comprising applying a pulse 900 and applying a pulse 920. Applying the pulse 900 causes a transition of the active region 18 into an amorphous phase to normalize the phase change material to resistance state 1000 of FIG. 10, and applying the pulse 920 causes a transition of a third portion of the active region 18 into a crystalline phase, thereby setting the phase change material 10 to a resistance within the resistive value distribution range of resistance state 1030 of FIG. 10. Preferable pulse 900 is similar to reset pulse 600.

In the illustrated embodiment the pulse 920 has a pulse length of time larger than that of the pulse 820 of FIG. 8, and is sufficient to raise the temperature of the third portion of the active region 18 above transition temperature 620 such that the third portion transitions into the crystalline phase. The larger pulse length results in the third portion of the phase change material that transitions into the crystalline phase being greater than the second portion, thus lowering the resistance of the phase change material to resistance state 1030 of FIG. 10. The length of time 930 between the completion of the quenching and the beginning of the pulse 920 is preferably small, and in some embodiments is equal to that of the time 730 of FIG. 7.

As mentioned above, the scope of the present invention includes programming the phase change material of a memory cell to one of three or more different data values. In the present invention, programming a data value to a memory cell includes applying a first pulse or a sequence of pulses to cause a transition of the active region of the phase change material into a “normalizing” phase, for example a generally amorphous phase which changes the pre-existing state of the memory cell into a known condition that is consistent in advance of the second pulse in the pulse pair. The second pulse therefore affects the memory cell in the same way, independent of its previous state. This transition of the active region “normalizes” the condition of the memory cell and helps to make the resulting resistances for each value more uniform, thus tightening the distribution of the resistances of the an array of memory cells.

In the embodiment illustrated in FIGS. 6-9, the presetting of a memory cell is accomplished with a single pulse that places that places an active region into a generally amorphous phase. Alternatively, the normalizing phase can be other than amorphous, including being generally crystalline. Additionally, the presetting of a memory cell can be accomplished using more than one pulse, for example using a pair of pulses.

Embodiments of memory cell 16 include phase change based memory materials, including chalcogenide based materials and other materials, for memory material 10. Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VI of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from column six of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te_(a)Ge_(b)Sb_(100-(a+b)), where a and b represent atomic percentages that total 100% of the atoms of the constituent elements. One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. (Ovshinsky '112 patent, cols 10-11.) Particular alloys evaluated by another researcher include Ge₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇. (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v. 3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky '112 at columns 11-13, which examples are hereby incorporated by reference.

Chalcogenides and other phase change materials are doped with impurities in some embodiments to modify conductivity, transition temperature, melting temperature, and other properties of memory elements using the doped chalcogenides. Representative impurities used for doping chalcogenides include nitrogen, silicon oxygen, silicon dioxide, silicon nitride, copper, silver, gold, aluminum, aluminum oxide, tantalum, tantalum oxide, tantalum oxide, tantalum nitride, titanium and titanium oxide. See, for example U.S. Pat. No. 6,800,504, and U.S. Patent Application Publication No. US 2005/0029502.

Phase change materials are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These phase change materials are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly.

Phase change materials can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state, and is referred to as a reset pulse. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state, and is referred to as a program pulse. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined empirically, without undue experimentation, specifically adapted to a particular phase change material and device structure.

Chalcogenide materials include Ge_(x)Sb_(y)Te_(z) where x:y:z=2:2:5, or other compositions with x: 0˜5; y: 0 ˜5; z: 0˜10. GeSbTe with doping, such as N—, Si—, Ti—, or other element doping may also be used.

GeSbTe may be formed by PVD sputtering or magnetron-sputtering method with reactive gases of Ar, N₂, and/or He, etc at the pressure of 1 mtorr˜100 mtorr. The deposition is usually done at room temperature. The collimater with aspect ratio of 1˜5 can be used to improve the fill-in performance. To improve the fill-in performance, the DC bias of several ten to several hundred volts is also used. On the other hand, the combination of DC bias and the collimater can be used simultaneously.

The post deposition annealing treatment with vacuum or N₂ ambient is sometimes needed to improve the crystallize state of chalcogenide material. The annealing temperature typically ranges 100° C. to 400° C. with an anneal time of less than 30 minutes.

The thickness of chalcogenide material depends on the design of cell structure. In general, a chalcogenide material with thickness of higher than 8 nm can have a phase change characterization so that the material exhibits at least two stable resistance states.

FIG. 11 is a simplified block diagram of an integrated circuit in accordance with an embodiment. The integrated circuit 1100 includes a memory array 1105 implemented using memory cells as described herein comprising programmable phase change material capable of being programmed to three or more different data values. A row decoder 1110 having read, set and reset modes is coupled to a plurality of word lines 1115 arranged along rows in the memory array 1105. A column decoder 1120 is coupled to a plurality of bit lines 1125 arranged along columns in the memory array 1105 for reading, setting and resetting memory cells in the memory array 1105. Addresses are supplied on bus 1160 to column decoder 1120 and row decoder 1110. Sense amplifiers and data-in structures in block 1130, including current sources for the read, set and reset modes, are coupled to the column decoder 1120 via data bus 1135. Data is supplied via the data-in line 1140 from input/output ports on the integrated circuit 1100 or from other data sources internal or external to the integrated circuit 1100, to the data-in structures in block 1130. In the illustrated embodiment, other circuitry 1165 is included on the integrated circuit 1100, such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by the phase change memory cell array. Data is supplied via the data-out line 1145 from the sense amplifiers in block 1130 to input/output ports on the integrated circuit 1100, or to other data destinations internal or external to the integrated circuit 1100.

A controller implemented in this example using bias arrangement state machine 1150 controls the application of bias arrangement supply voltages and current sources 1155, such as read, program pulse pairs and verify pulses described herein, comprising voltages and/or currents for the word lines and bit lines, and controls the word line/source line operation using an access control process. The controller can be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, the controller comprises a general-purpose processor, which may be implemented on the same integrated circuit, which executes a computer program to control the operations of the device. In yet other embodiments, a combination of special-purpose logic circuitry and a general-purpose processor may be utilized for implementation of the controller.

The above descriptions may have used terms such as above, below, top, bottom, over, under, et cetera. These terms are used to aid understanding of the invention are not used in a limiting sense.

While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.

Any and all patents, patent applications and printed publications referred to above are incorporated by reference. 

1. A memory device comprising: a memory cell comprising phase change material; and bias circuitry adapted to apply a bias arrangement to the memory cell for storing a data value; wherein the bias arrangement comprises a first pulse pair if the data value is a first data value, the first pulse pair comprising a first pulse to cause a transition of an active region into an amorphous phase, and a second pulse to cause a transition of a first portion of the active region into a crystalline phase, thereby setting the phase change material to a resistance corresponding to the first data value; wherein the bias arrangement comprises a second pulse pair if the data value is a second data value, the second pulse pair comprising a third pulse to cause a transition of the active region into the amorphous phase, and a fourth pulse to cause a transition of a second portion of the active region into a crystalline phase, thereby setting the phase change material to a resistance corresponding to the second data value.
 2. The memory device of claim 1, wherein the first pulse and third pulse cause transitions of the active region into the amorphous phase independent of a previous state of the phase change material.
 3. The memory device of claim 1, wherein the bias arrangement consists of the first pulse pair if the data value is the first data value, and consists of the second pulse pair if the data value is the second data value.
 4. The memory device of claim 1, wherein the bias arrangement comprises a third pulse pair if the data value is a third data value, the third pulse pair comprising a fifth pulse to cause a transition of a portion of the active region into the amorphous phase, and a sixth pulse to cause a transition of a remaining portion of the active region into the amorphous phase, thereby setting the phase change material to a resistance corresponding to the third data value.
 5. A memory device comprising: a memory cell comprising phase change material; and bias circuitry adapted to apply a bias arrangement to the memory cell for storing a data value; wherein the bias arrangement comprises a first pulse pair if the data value is a first data value, the first pulse pair comprising a first pulse to preset the phase change material to a normalizing resistance state, and a second pulse to set the phase change material from the normalizing resistance state to a resistance corresponding to the first data value; wherein the bias arrangement comprises a second pulse pair if the data value is a second data value, the second pulse pair comprising a third pulse to preset the phase change material to the normalizing resistance state, and a fourth pulse to set the phase change material from the normalizing resistance state to a resistance corresponding to the second data value, wherein the fourth pulse has a pulse length of time larger than that of the second pulse.
 6. The memory device of claim 5, wherein a resistance in the normalizing resistance state is less than a resistance corresponding to the first data value, and less than a resistance corresponding to the second data value.
 7. The memory device of claim 5, wherein the phase change material has an active region in a generally crystalline phase in the normalizing resistance state. 